Lessons learned from a long-term manipulation experiment in a semi-arid savanna ecosystem

Semi-arid ecosystems dominate the interannual variability and trend of the terrestrial carbon sink. They are sensitive to anthropogenic environmental changes, including shifts in the nitrogen (N) to phosphorus (P) ratio driven by increasing N deposition.

In 2015, a large-scale fertilization experiment was established at Majadas de Tiétar, a tree-grass ecosystem in western Spain. Three eddy covariance towers operate simultaneously at the site: one serves as unfertilized control plot, one measures an area fertilized with N, and the third samples an area with N and P addition. This setup provides an exceptional opportunity to study the long-term influence of altered N:P ratios on ecosystem functioning. Flux measurements are complemented by a variety of other instruments, such as lysimeters, mini-rhizotrons, soil chambers, soil sensors, phenocams and proximal sensing instruments. The comprehensive measurement setup at Majadas de Tiétar therefore enables a deeper understanding of the trends and interactions among climate change, nutrient availability and the biogeochemical cycles of carbon, N, and P in semi-arid ecosystems.

We found that both fertilization schemes increased carbon uptake, and that N+P addition enhanced the water use efficiency more than N-only addition. Fertilization also increased the inter-annual variability of net ecosystem exchange (NEE) and altered the sensitivity of seasonal NEE to its drivers. However, water limitation in summer and energy limitation in winter overweighed fertilization effects at the seasonal scale.